Various ink-jet recording methods such as a method of discharging an ink by utilizing electrostatic attracting force (electric-field controlling method), a method of discharging an ink by utilizing driving pressure of a piezoelectric element (drop-on-demand method or pressure pulse method), and a method of discharging an ink by utilizing pressure generated by forming gas bubbles with high heat and growing the same (bubble or thermal jet method) are known as an ink-jet recording method. An extremely high definition image can be obtained by those methods. Furthermore, in those ink-jet recording methods, a water-based ink using water as the main solvent and an oil-based ink using an organic solvent as the main solvent are generally used.
A printed image obtained using a water-based ink is generally poor in water resistance, and it is difficult to print an image on a recording medium having water-resistant surface. Contrary to this, an oil-based ink has various advantages such that it can provide a printed image having excellent water resistance, and additionally, it facilitates printing on a recording medium having water-resistant surface and a high-quality paper. Furthermore, an oil-based ink using a pigment as a coloring material has excellent light resistance. Even though those advantages, further improvement in properties such as fixability, durability, wear resistance and curability has been desired with the expansion of applications of the oil-based ink. In particular, where the coloring material is white, further improvement in properties such as fixability, durability, wear resistance and curability is desired, in addition to improvement of image in terms of whiteness and shielding property.
A technique in which a resin which dissolves in a solvent is used and the resin bonds and fixes a pigment and a recording medium at the time when the solvent is dried, is disclosed as a method of increasing fixability of a pigment to a recording medium (for example, see Patent Document 1). The larger the molecular weight of the resin which dissolves in a solvent, the more excellent the fixability and strength when dried, but this is accompanied with a great increase of the viscosity of an ink composition. As a result, discharge property of an ink composition is decreased, and in an extreme case, a nozzle is clogged and an ink composition cannot be discharged. A resin having a low molecular weight can suppress the increase of viscosity, but fixability and strength of an ink composition are decreased. Additionally, where the amount of a resin present in a solvent in a dissolved state is excess, it induces discharge defects such as flight deflection and scattering, regardless of a molecular weight of a resin.
In view of the above, a technique of adding an emulsion or a suspension, having resin fine particles dispersed in a dispersion medium (hereinafter simply referred to as a “resin emulsion” without distinguishing an emulsion and a suspension), to an ink composition is proposed (for example, see Patent Documents 2 to 5). However, unlike a water-based ink composition comprising water as the main component, an oil-based ink composition greatly changes its properties depending on the kind of a solvent used. Therefore, it is necessary to appropriately select a resin emulsion suitable to the kind of a resin emulsion and a dispersion medium used.
Patent Document 1: WO02/100652
Patent Document 2: JP-A-2004-250659
Patent Document 3: JP-A-2005-23299
Patent Document 4: JP-A-2005-290362
Patent Document 5: JP-A-2005-314655